1. Field of the Invention
This invention relates to a molding method capable of molding a glass product with high precision of a fine structure without creating molding burrs, even if the glass product has a weak symmetry as of an optical fiber fixing member and the fine structure such as an optical fiber engagement portions.
2. Description of Related Art
An optical fiber used for optical communication is generally a fine glass fiber. For example, a quartz single mode optical fiber used for long distance optical communication is constituted of a core having an outer diameter of about 10 micron meters and a clad covering the core and having an outer diameter of 125 micron meters. A quartz multi-mode optical fiber is constituted of a core having an outer diameter of 50 to 100 micron meters and a clad covering the core and having an outer diameter of 125 micron meters. Accordingly, high alignment precision is required to reduce a connection loss at optical connecting points when optical fibers are optically connected to each other or when an optical fiber is optically connected to an optical device such as an optical waveguide, a lens, an light emitting device, a photo-receiving device, etc. In particular, optical connections between quartz single mode optical fibers and between a quartz single mode optical fiber and a quartz glass single mode optical waveguide require a high alignment precision of around .+-.1 micron meter.
To optically connect an optical fiber with another optical fiber or optical device, the optical fiber is fixed in advance by an optical fiber fixing member such as an optical connector or optical fiber array. The optical fiber array here means a member at least including an optical fiber guide block and a fiber fixing lid. The optical fiber guide block is made of a thin plate formed with engagement portions for fixing optical fibers to position the optical fibers. The fiber fixing lid is made of a thin plate for pressing the optical fibers engaging with the engagement member to fix the optical fibers. For example, Japanese Unexamined Patent Publication, Heisei No. 7-5,341 discloses an optical fiber array for fixing a tape fiber in which plural optical fibers arranged in a row are protected by a resin cover. This optical fiber array is shown in FIG. 7.
As shown in FIG. 7, the optical fiber array 200 includes an optical fiber guide block 204 and a fiber fixing lid for optical fibers 205. The optical fiber guide block 204 is a block in a thin plate shape on which a prescribed number of V-shaped grooves 203 serving as engagement portions for fixing optical fibers are formed for fixing optical fibers 202 striped from a tape fiber 201. The fiber fixing lid 205 for optical fibers is a block in a thin plate shape to press the optical fibers to fix the optical fibers 202 engaged with the V-shaped grooves 203. The optical fiber guide block 204 constituting the optical fiber array 200 has, in addition to the V-shaped grooves 203, a seat 207 for fixing a covered portion 206 of the tape fiber 201. The seat 207 is formed at a position lower than the V-shaped grooves 203. The optical fiber array 200 includes a fiber fixing lid 208 having a prescribed cross section to securely hold the covered portion 206 fixed at the seat 207.
To optically connect optical fibers fixed by optical fiber fixing members such as optical connectors and optical fiber arrays with each other or connect optical fibers fixed by an optical fiber fixing member with an optical device under a high alignment precision, an active alignment is conventionally used in using a precision stage. The active alignment is implemented in the following manner, for optical fibers fixed by optical fiber arrays and connected with each other,
First, an optical fiber array to which optical fibers are fixed (hereinafter referred to as "optical fiber array A") is fixed to a holder on a precision stage, and another optical fiber array to which optical fibers are fixed (hereinafter referred to as "optical fiber array B") is fixed to another holder on the precision stage. Light is made to enter in an optical fiber fixed on the optical fiber array A from an optical fiber end in opposition to an optical connection side end (end positioned on a side where the optical fiber array is connected to another optical fiber array or optical device, between the opposing ends of the optical fiber) of the optical fiber array A, and an optical detector is set at an end located in opposition to the optical connection side end in the optical fiber array B. Then, the precision stage is scanned over a wide range to explore a position at which the optical detector detects optical power even of a small amount (this stage is referred to as "first step"). Then, the precision stage is scanned by a very small distance as to pick up the maximum optical power by the optical detector, thereby obtain the aimed high precision alignment (this stage is referred to as "second step").
Because a considerable time is required to scan the stage over the wide range at the first step during the active alignment, it is desirable to substantially complete the first step when the optical fiber fixing member is fixed to the stage's holder to make the high precision alignment easy. To do so, it is desired that the optical fiber engagement portions on the optical fiber fixing member is built with high precision, as well as that the optical fiber engagement portions are built with high precision such that positional precision, when measured in reference with the bottom face or side face of the optical fiber fixing member, are within 1/10 or less of the core diameter of the optical fibers to optically connected in use of the optical fiber fixing member. For example, in the case where quartz single mode optical fibers having a core diameter of around 10 micron meters are optically connected to each other, or in the case where a quartz single mode optical fiber is connected to a quartz glass single mode optical waveguide, the positional precision is desirably within 10 micron meters or less, and if the positional precision is within 5 micron meters or less, the alignment can be done easily.
When the position precision is reduced to about 1/10 or less of the core diameter of the optical fibers, the fibers can be optically connected by a passive alignment The passive alignment is an alignment method for mechanically adjusting the positions of the optical fiber fixing members or the fixing member and an optical device, using the side or bottom faces of the optical fiber fixing members as reference faces without detecting light entering into and emerging from the optical fibers. Thus, the optical fiber fixing members such as optical fiber arrays are required to have a high precision not only of the engagement portions for optical fibers for fixing the arranged optical fibers but also of the side or bottom face used as reference faces for alignment.
Glass, ceramic, silicon, resin, etc. are used as a material constituting a member for fixing optical fibers (hereinafter referred to as "optical fiber fixing member") such as optical fiber array or the like. Ultraviolet ray setting type adhesives having good property for work are desirable for fixing the fiber fixing lid on the optical fiber guide block and for connecting the optical fiber array with other optical fiber array or the like. Therefore, glasses having good ultraviolet transparency are getting favored as a material for optical fiber arrays. An optical fiber guide block required to have a high precision in size at optical fiber engagement portions, among optical fiber fixing members, has been fabricated by mechanically processing a glass block and the like in use of a dicing saw, diamond hone, etc. Such a fabrication process, however, raises a problem about mass production, production costs, and yields.
A method applying a method for molding optical glass lens has been known as a mass production method for optical fiber fixing members with lower costs. For example, Japanese Unexamined Patent Publication, Heisei No. 6-201,936 discloses a method for pressing a transparent material such a glass plate or the like with a high temperature by a mold having projections for forming grooves. Japanese Unexamined Patent Publication Heisei No. 7-218,739 discloses that a pitch precision at a molded optical fiber engagement portions is within 0.5 micron meter or less, a high precision. Japanese Unexamined Patent Publication Heisei No. 8-211,244 discloses a molding method for optical fiber fixing member using a glass containing no lead and having a low softening point.
Any of those methods is for molding an optical fiber fixing member. Those publications, however, do not disclose any means for improving precision of optical fiber engagement portions for fixing optical fibers orderly placed and of a side or bottom face as a reference face for alignment, and merely use a method, no more than a converted method for molding optical glass lenses.
Many optical fiber fixing member have a thin plate shape having a rectangular form when viewed from the top. Optical fiber engagement portions are formed at a portion of the thin plate and have a gap portion used as a seat. The optical fiber fixing members thus require a high molding precision, though it is very hard to mold such a fixing member in comparison with molding of lenses because the optical fiber fixing members thus have a shape of a weak symmetry. Therefore, even if a conventional molding technique for molding optical glass lenses applies for molding fixing members as it is, a useful optical fiber fixing member may not be molded. Particularly, to make higher the precision of the side or bottom face serving as reference faces for alignment, it is required to suppress molding burrs from occurring. The publications above, however, contain no disclosure about a method for suppressing molding burrs. In molding of optical lenses, some proposal has a method for suppressing molding burrs. However, as described below, those methods cannot be used, as they are, for molding optical fiber fixing members.
In regard with molding of an optical lens, it has been known that, as disclosed in Japanese Unexamined Patent Publication No. 60-118,642, substantially cylinder or sphere glass is used as a molding preform, and a mold including an upper mold, a lower mold, and a side mold is used. When such a glass preform is heated at a molding temperature and molded with pressure, the glass preform is extended coaxially and uniformly to fill the cavity of the mold. Setting the volume of the glass preform to a volume a little smaller than the cavity may prevent glass's encroachments into clearances between the side mold and the upper and lower mold, or namely, molding burrs. Moreover, as shown in FIG. 4 in the Publication, to make management for the glass preform's volume easy, some glass escaping portions are formed at portions optically not raising any problem, thereby preventing molding burrs.
In Japanese Patent Publication Heisei No. 6-15,414, and Japanese Patent Publication Heisei No. 6-17,240, disclosed are methods for molding glass lenses in preventing molding burrs by providing temperature differences among respective molds, the upper, lower, and side molds. Giving temperature differences differentiates glass transformation speeds at contact portions of the upper and lower molds, and controlling the glass' volume at a fixed amount or less brings glass unfilled portions raising no optical problem and consequently prevents molding burrs from occurring.
Japanese Unexamined Patent Publication, Showa No. 62-252,331 discloses a molding method for glass lens in which a mold material having a relation that thermal expansion coefficient of the glass material is larger than the thermal expansion coefficient of the mold material, which is larger than the thermal expansion coefficient of the side mold. According to this method, the thermal expansion coefficient differences between the mold and side mold makes the clearances between the side mold and the upper and lower molds smaller than those at a room temperature, thereby preventing molding burrs.
Any of such molding burr suppressing methods known for molding methods for glass lens has a premise that the glass preform spreads in the mold cavity uniformly. That is, the molding burrs can be prevented because the optical lens is a rotary symmetric body, and therefore, the horizontal cross section of the side mold is circle, and because unfilled portion of the glass can be made around clearances adjacent to the side mold.
Optical fiber fixing members, however, have weak symmetry in shapes. Therefore, it is unlikely that the glass spreads without contacting to the side mold at all. For example, even if a glass preform having a proximate shape to the optical fiber fixing member is used, edges, though originally the rectangular glass preform, may be rounded during application of pressure and expand horizontally to transform the preform. Therefore, for example, the glass at a projected portion can reach a side mold wall due to pressure and may cause molding burrs, but at the other portions the glass may create an unfilled situation in which the glass does not reach the side mold wall. If a whole form is made up by filling the glass even to the unfilled portions, the molding burrs are made more larger. Uneven glass extension becomes more outstanding as the products have weaker symmetry, and when the thickness of the products is not uniform, likewise a gap at the seat, such uneven extension becomes more outstanding. The molding burrs thus created may result in glass garbage, tending to raise problems when the product is produced massively. Although a trimming process of molding burrs is possible in a technical sense, it is not favorable because it may cause higher costs.
As described above, the optical fiber fixing member is required to be molded with a high precision with respect to any of the molded face for optical fiber engagement portions, the side face, and the bottom face. Particularly, the optical fiber engagement portions are required to elevate the molding precision on the side of the optical connection side end as within .+-.1 micron meter or less to reduce optical connection loss. To increase molding precision, if the glass is filled more into the mold cavity, more molding burrs may be created. If the optical connection side end has molding burrs, the optical fiber would be mounted on the molding burrs when inserted, so that the optical fiber cannot be aligned on the optical fiber engagement portion with high precision. The cross-sectional shape of the optical connection side end of the optical fiber fixing member desirably has a precise rectangular shape to improve alignment precision and to fix the end with other optical device by adhesive. If the edge of the molded face of the optical fiber engagement portions has molding burrs, however, the fiber fixing lid may be mounted on the molding burrs, thereby preventing the optical fibers from fixed with a high precision. If the corner on a side of the bottom has molding burrs, the optical fiber fixing member cannot be fixed to the holder for alignment, or precision stage's holder, with a high precision, thereby disabling the alignment process. As described above, for the optical fiber fixing member, any molding burr at almost all corners of the optical fiber fixing member should not be allowed while spaces around the optical connection side end have to be filled as mush as possible.
Thus, in molding items not formed of a rotary symmetric body, such as optical fiber fixing members, it is unlikely that the pressurized glass contacts uniformly to the molding face of the side mold, even though the glass must be filled in spaces around the optical connection side end. Hence, a molding technique is required in which no molding burr occurs even while the glass is filled well in spaces around the optical connection side end by raising the glass' filling degree in the mold cavity. Furthermore, the optical fiber fixing member has a fine structure at the optical fiber engagement portions. The fine structure is required to be formed with a high precision, and glass filling impairments in the optical fiber engagement portions would result in a fatal defect. A molding technique capable of molding such a fine structure of the optical fiber fixing member is especially required notwithstanding high or low glass' filling degree in the mold cavity.